Haven't seen too many comparisons, but there is this one showing both GTX and the 640MB GTS benched both in single and SLI modes: Link
It varies with the title, but the trend seems to be that the GTX performs about halfway between the single and SLI speeds of the GTS. Additionally, SLI in most games yields far less than the 40-50% performance increase you might hope for.
On a pragmatic note, the variance in costs (2 GTS v. 1 GTX) seems to coincide neatly with the performance. For myself, that amount of extra performance isn't worth the extra troubleshooting, noise, heat, and power consumption.
It varies with the title, but the trend seems to be that the GTX performs about halfway between the single and SLI speeds of the GTS. Additionally, SLI in most games yields far less than the 40-50% performance increase you might hope for.
On a pragmatic note, the variance in costs (2 GTS v. 1 GTX) seems to coincide neatly with the performance. For myself, that amount of extra performance isn't worth the extra troubleshooting, noise, heat, and power consumption.